Power reduction in combustible gas sensors

ABSTRACT

A system includes a primary combustible gas sensor and a trigger combustible gas sensor including a first trigger element of low-thermal-mass which includes a first trigger heating element in operative connection with electronic circuitry. The trigger combustible gas sensor also includes a second trigger element of low thermal mass including a second trigger heating element. The second trigger element is also in operative connection with the electronic circuitry. The electronic circuitry further has a first trigger mode of operating in which the first trigger element is heated to a temperature at or above a temperature at which the first trigger element causes combustion of the at least one combustible gas analyte and wherein the second trigger element is operated as a trigger compensating element. The electronic circuitry is configured to operate the trigger combustible gas sensor to detect a value of a response at or above a threshold value. The primary combustible gas sensor is activated from a low-power state upon the threshold value being detected by the trigger combustible gas sensor.

BACKGROUND

The following information is provided to assist the reader inunderstanding technologies disclosed below and the environment in whichsuch technologies may typically be used. The terms used herein are notintended to be limited to any particular narrow interpretation unlessclearly stated otherwise in this document. References set forth hereinmay facilitate understanding of the technologies or the backgroundthereof. The disclosure of all references cited herein are incorporatedby reference.

Catalytic or combustible (flammable) gas sensors have been in use formany years to, for example, prevent accidents caused by the explosion ofcombustible or flammable gases. In general, combustible gas sensorsoperate by catalytic oxidation of combustible gases.

The operation of a catalytic combustible gas sensor proceeds throughelectrical detection of the heat of reaction of a combustible gas on theoxidation catalysts, usually through a resistance change. The oxidationcatalysts typically operate in a temperature above 100° C. (and, moretypically, above 300° C.) to catalyze combustion of an analyte (forexample, in the range of 350 to 600° C. temperature range for methanedetection). Therefore, the sensor must sufficiently heat the sensingelement through resistive heating. In a number of combustible gassensors, the heating and detecting element are one and the same andcomposed of a platinum alloy because of its large temperaturecoefficient of resistance and associated large signal in target/analytegas. The heating element may be a helical coil of fine wire or a planarmeander formed into a hotplate or other similar physical form. Thecatalyst being heated often is an active metal catalyst dispersed upon arefractory catalyst substrate or support structure. Usually, the activemetal is one or more noble metals such as palladium, platinum, rhodium,silver, and the like and the support structure is a refractory metaloxide including, for example, one or more oxides of aluminum, zirconium,titanium, silicon, cerium, tin, lanthanum and the like. The supportstructure may or may not have high surface area (that is, greater than75 m²/g). Precursors for the support structure and the catalytic metalmay, for example, be adhered to the heating element in one step orseparate steps using, for example, thick film or ceramic slurrytechniques. A catalytic metal salt precursor may, for example, be heatedto decompose it to the desired dispersed active metal, metal alloy,and/or metal oxide.

As illustrated in FIGS. 1A and 1B, a number of conventional combustiblegas sensors such as illustrated sensor 10 typically include an elementsuch as a platinum heating element wire or coil 20 encased in arefractory (for example, alumina) bead 30, which is impregnated with acatalyst (for example, palladium or platinum) to form an active orsensing element, which is sometimes referred to as a pelement 40,pellistor, detector or sensing element. A detailed discussion ofpelements and catalytic combustible gas sensors which include suchpelements is found in Mosely, P. T. and Tofield, B. C., ed., Solid StateGas Sensors, Adams Hilger Press, Bristol, England (1987). Combustiblegas sensors are also discussed generally in Firth, J. G. et al.,Combustion and Flame 21, 303 (1973) and in Cullis, C. F., and Firth, J.G., Eds., Detection and Measurement of Hazardous Gases, Heinemann,Exeter, 29 (1981).

Bead 30 will react to phenomena other than catalytic oxidation that canchange its output (i.e., anything that changes the energy balance on thebead) and thereby create errors in the measurement of combustible gasconcentration. Among these phenomena are changes in ambient temperature,humidity, and pressure.

To minimize the impact of secondary effects on sensor output, the rateof oxidation of the combustible gas may, for example, be measured interms of the variation in resistance of sensing element or pelement 40relative to a reference resistance embodied in an inactive, compensatingelement or pelement 50. The two resistances may, for example, be part ofa measurement circuit such as a Wheatstone bridge circuit as illustratedin FIG. 1C. The output or the voltage developed across the bridgecircuit when a combustible gas is present provides a measure of theconcentration of the combustible gas. The characteristics ofcompensating pelement 50 are typically matched as closely as possiblewith active or sensing pelement 40. In a number of systems, compensatingpelement 50 may, however, either carry no catalyst or carry aninactivated or poisoned catalyst. In general, changes in properties ofcompensating elements caused by changing ambient conditions are used toadjust or compensate for similar changes in the sensing element.

Active or sensing pelement 40 and compensating pelement 50 can, forexample, be deployed within wells 60 a and 60 b of an explosion-proofhousing 70 and can be separated from the surrounding environment by aflashback arrestor, for example, a porous metal frit 80. Porous metalfrit 80 allows ambient gases to pass into housing 70 but preventsignition of flammable gas in the surrounding environment by the hotelements. Such catalytic gas sensors are usually mounted in instrumentswhich, in some cases, must be portable or wireless and, therefore, carrytheir own power supply. It is, therefore, desirable to minimize thepower consumption of a catalytic gas sensor.

Oxidation catalysts formed onto a helical wire heater are typicallyreferred to as pelements, while those formed onto hotplates (whethermicroelectronic mechanical systems (MEMS) hotplates or conventional,larger hotplates) are sometimes known by the substrate. Oxidativecatalysts formed on MEMS heating elements are sometimes referred toherein as MEMS pellistors. As described above, the detecting pelementsor catalytically active hotplates can be paired with a similarly sizedheater coated with materials with similar thermal conductivity as theactive catalyst but without active sites. The inactive pelement orhotplate may be used to compensate for changes in ambient temperature,relative humidity, or background thermal conductivity not associatedwith a combustible gas and are therefore often referred to ascompensators. The matched pair of detecting and compensating elementscan be assembled in a Wheatstone bridge configuration for operation andcombustible gas detection, which requires that both the detector andcompensator operate at the same elevated temperature. Thehigh-temperature operation of the catalytic sensing element requires asignificant amount of power consumption. Power consumption isparticularly a problem in the case of detecting combustible gases asdetection should be performed very often or continuously to ensure asafe environment. Portable instrument and wireless installations rely onbattery systems for power.

In a number of currently available combustible gas sensors which includesensing and compensating beads, a single, third bead, sometimes referredto as a trigger or a “sniff bead”, is included which requires verylittle power. The trigger bead is not required to provide a linearresponse and is not required to be immune to positive excursiontemperature/humidity fluctuations. With the restrictions of linearityand immunity to positive temperature and humidity fluctuationseliminated, the size and power restrictions of the “trigger” bead may bereduced in comparison to detector beads.

SUMMARY

In one aspect, a sensor system includes electronic circuitry including acontrol system and a primary combustible gas sensor. The primarycombustible gas sensor includes a first primary element in operativeconnection with the electronic circuitry. The first primary elementincludes a first primary support structure, a first primary catalystsupported on the first primary support structure and a first primaryheating element in operative connection with the first primary supportstructure. The first primary combustible gas sensor further includes asecond primary element in operative connection with the electroniccircuitry. The second primary element includes a second primary supportstructure, a second primary catalyst supported on the second primarysupport structure and a second primary heating element in operativeconnection with the second primary support structure.

The sensor system further includes a trigger combustible gas sensorincluding a first trigger element of low-thermal-mass which includes afirst trigger heating element in operative connection with theelectronic circuitry. The trigger combustible gas sensor also includes asecond trigger element of low thermal mass including a second triggerheating element. The second trigger element is also in operativeconnection with the electronic circuitry. The electronic circuitryfurther has a first trigger mode of operating in which the first triggerelement is heated to a temperature at or above a temperature at whichthe first trigger element causes combustion of the at least onecombustible gas analyte and wherein the second trigger element isoperated as a trigger compensating element.

The electronic circuitry is further configured to operate the triggercombustible gas sensor to detect a value of a response at or above athreshold value. The primary combustible gas sensor is activated (from alow-power state) upon the threshold value being detected by the triggercombustible gas sensor. As used herein, a low-power state includes azero-power state.

The electronic circuitry may, for example, have a first primary mode ofoperating in which the first primary element is heated above atemperature at which the first primary catalyst catalyzes combustion ofat least one combustible gas analyte and wherein the second primaryelement is operated as a primary compensating element.

The second primary element may, for example, be operated at a lowerpower than the first primary element in the first primary mode. In anumber of embodiments, the electronic circuitry has a second primarymode of operating in which the second primary element is heated above atemperature at which the first primary catalyst catalyzes combustion ofat least one combustible gas analyte and wherein the first primaryelement is operated as a primary compensating element. The first primaryelement may, for example, be operated at a lower power than the secondprimary element in the second primary mode.

In a number of embodiments, the second trigger element is operated at alower power than the first trigger element in the first trigger mode.The electronic circuitry may, for example, have a second trigger mode ofoperating in which the second trigger element is heated above atemperature at which the second trigger element causes combustion of atleast one combustible gas analyte and wherein the first trigger elementis operated as a trigger compensating element. The first trigger elementmay, for example, be operated at a lower power than the second triggerelement in the second trigger mode.

In a number of embodiments, the first trigger element includes a firsttrigger catalyst, and the second trigger element includes a secondtrigger catalyst.

In a number of embodiments, the first trigger element includes a firstMEMS element and the second trigger element includes a second MEMSelement. In a number of embodiments, the value of the response of thetrigger combustible gas sensor is a concentration of the at least onecombustible gas analyte which is output via a user interface inoperative connection with the control system.

In a number of embodiments, during the first trigger mode, theelectronic circuitry is configured to periodically cycle the firsttrigger element between the temperature at or above which the firsttrigger element causes combustion of the at least one combustible gasanalyte and another temperature at which the first trigger element issubstantially inactive to cause combustion of the at least onecombustible gas analyte. In a number of embodiments, during the secondtrigger mode, the electronic circuitry is configured to periodicallycycle the second trigger element between the temperature at or abovewhich the second trigger element causes combustion of the at least onecombustible gas analyte and another temperature at which the secondtrigger element is substantially inactive to cause combustion of the atleast one combustible gas analyte.

A user may, for example, be notified via a user interface in operativeconnection with the control system when the value of the response of thetrigger combustible gas sensor is at or above the threshold value. In anumber of embodiments, each of the first trigger element and the secondtrigger element independently has a thermal time constant no greaterthan 8 seconds, no greater than 1 second or no greater than 0.250seconds.

In another aspect, a sensor system includes electronic circuitryincluding a control system and a primary combustible gas sensor. Theprimary combustible gas sensor includes a first primary element inoperative connection with the electronic circuitry. The first primaryelement includes a first primary support structure, a first primarycatalyst supported on the first primary support structure and a firstprimary heating element in operative connection with the first primarysupport structure. The first primary element further includes a secondprimary element in operative connection with the electronic circuitry.The second primary element includes a second primary support structure,a second primary catalyst supported on the second primary supportstructure and a second primary heating element in operative connectionwith the second primary support structure.

The sensor system further includes a trigger combustible gas sensorincluding a first MEMS trigger element comprising a first triggerheating element. The first trigger MEMS element is in operativeconnection with the electronic circuitry. The electronic circuitry isconfigured to heat the first MEMS trigger element to at temperature ator above a temperature at which the first MEMS trigger element causescombustion of the at least one combustible gas analyte. The electroniccircuitry is further configured to operate the trigger combustible gassensor to detect a value of a response at or above a threshold value.The primary combustible gas sensor is activated (from a low-power state)upon the threshold value being detected by the trigger combustible gassensor.

As described above, the electronic circuitry may, for example, have afirst primary mode of operating in which the first primary element isheated to a temperature at or above a temperature at which the firstprimary catalyst catalyzes combustion of at least one combustible gasanalyte and wherein the second primary element is operated as a primarycompensating element.

In a number of embodiments, the trigger combustible gas sensor furtherincludes a second MEMS trigger element including a second triggerheating element. The second MEMS trigger element is in operativeconnection with the electronic circuitry. The second MEMS triggerelement is operated as a compensating element in at least one mode ofoperation of the sensor system. The electronic circuitry may, forexample, be configured to periodically cycle the first trigger elementbetween the temperature at or above which the first trigger elementcauses combustion of the at least one combustible gas analyte andanother temperature at which the first trigger element is substantiallyinactive to cause combustion of the at least one combustible gasanalyte. The electronic circuitry may, for example, be configured toperiodically cycle the first trigger element between the temperature ator above which the first trigger element causes combustion of the atleast one combustible gas analyte and another temperature at which thefirst trigger element is substantially inactive to cause combustion ofthe at least one combustible gas analyte. In a number of embodiments,each of the first MEMS trigger element and the second MEMS triggerelement independently has a thermal time constant no greater than 1second, no greater than 0.500 seconds or no greater than 0.250 seconds.

In a further aspect, a method of sensing at least one combustible sensorsystem, includes providing a system including a control system and aprimary combustible gas sensor. The primary combustible gas sensorincludes a first primary element in operative connection with theelectronic circuitry. The first primary element includes a first primarysupport structure, a first primary catalyst supported on the firstprimary support structure and a first primary heating element inoperative connection with the first primary support structure. The firstprimary combustible gas sensor further includes a second primary elementin operative connection with the electronic circuitry. The secondprimary element includes a second primary support structure, a secondprimary catalyst supported on the second primary support structure and asecond primary heating element in operative connection with the secondprimary support structure. The system further includes a triggercombustible gas sensor including a first trigger element oflow-thermal-mass which includes a first trigger heating element inoperative connection with the electronic circuitry. The triggercombustible gas sensor also includes a second trigger element of lowthermal mass including a second trigger heating element. The secondtrigger element is also in operative connection with the electroniccircuitry. The electronic circuitry further has a first trigger mode ofoperating in which the first trigger element is heated to a temperatureat or above a temperature at which the first trigger element causescombustion of the at least one combustible gas analyte and wherein thesecond trigger element is operated as a trigger compensating element.

The method further includes operating the electronic circuitry in afirst trigger mode in which the first trigger element is heated to atemperature at or above a temperature at which the first trigger elementcauses combustion of the at least one combustible gas analyte andwherein the second trigger element is operated as a trigger compensatingelement and activating the primary combustible gas sensor from alow-power state via the electronic circuitry if the trigger combustiblegas sensor detects a value of a response at or above a threshold value.

The method may further include, upon activating the primary combustiblegas sensor, operating the electronic circuitry in a first primary modein which the first primary element is heated above a temperature atwhich the first primary catalyst catalyzes combustion of at least onecombustible gas analyte and wherein the second primary element isoperated as a primary compensating element. The sensor system may, forexample, further be operated as described above.

In still a further aspect, a method includes providing a sensor systemincluding electronic circuitry including a control system and a primarycombustible gas sensor. The primary combustible gas sensor includes afirst primary element in operative connection with the electroniccircuitry. The first primary element includes a first primary supportstructure, a first primary catalyst supported on the first primarysupport structure and a first primary heating element in operativeconnection with the first primary support structure. The first primaryelement further includes a second primary element in operativeconnection with the electronic circuitry. The second primary elementincludes a second primary support structure, a second primary catalystsupported on the second primary support structure and a second primaryheating element in operative connection with the second primary supportstructure. The sensor system further includes a trigger combustible gassensor including a first MEMS trigger element comprising a first triggerheating element. The first trigger MEMS element is in operativeconnection with the electronic circuitry. The electronic circuitry isconfigured to heat the first MEMS trigger element to at temperature ator above a temperature at which the first MEMS trigger element causescombustion of the at least one combustible gas analyte. The electroniccircuitry is further configured to operate the trigger combustible gassensor to detect a value of a response at or above a threshold value.The primary combustible gas sensor is activated (from a low-power state)upon the threshold value being detected by the trigger combustible gassensor.

The method further includes operating the electronic circuitry to heatthe first MEMS trigger element to at temperature at or above atemperature at which the first MEMS trigger element causes combustion ofthe at least one combustible gas analyte and activating the primarycombustible gas sensor from a low-power state via the electroniccircuitry upon the threshold value being detected by the triggercombustible gas sensor.

The method may further include, upon activating the primary combustiblegas sensor, operating the electronic circuitry in a first primary modein which the first primary element is heated to a temperature at orabove a temperature at which the first primary catalyst catalyzescombustion of at least one combustible gas analyte and wherein thesecond primary element is operated as a primary compensating element.The sensor system may further be operated as described above.

The present devices, systems, and methods, along with the attributes andattendant advantages thereof, will best be appreciated and understood inview of the following detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates schematically an embodiment of a conventionalcombustible gas sensor.

FIG. 1B illustrates schematically an enlarged view of the sensingelement of the combustible gas sensor of FIG. 1A.

FIG. 1C illustrates a schematic view of a Wheatstone bridge circuitincorporating the sensing element and the compensating element of thecombustible gas sensor of FIG. 1A.

FIG. 2A illustrates schematically a cross-sectional view of anembodiment of a low-thermal mass combustible gas sensor suitable for useherein.

FIG. 2B illustrates a perspective view of the low-thermal-masscombustible gas sensor of FIG. 2A in operative connection with a printedcircuit board.

FIG. 2C illustrates schematically a cross-sectional view of a systemhereof including a conventional combustible gas sensor and a combustiblegas sensor including a sensing element and a compensating element of lowthermal mass.

DETAILED DESCRIPTION

It will be readily understood that the components of the embodiments, asgenerally described and illustrated in the figures herein, may bearranged and designed in a wide variety of different configurations inaddition to the described representative embodiments. Thus, thefollowing more detailed description of the representative embodiments,as illustrated in the figures, is not intended to limit the scope of theembodiments, as claimed, but is merely illustrative of representativeembodiments.

Reference throughout this specification to “one embodiment” or “anembodiment” (or the like) means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearance of the phrases “in oneembodiment” or “in an embodiment” or the like in various placesthroughout this specification are not necessarily all referring to thesame embodiment.

Furthermore, described features, structures, or characteristics may becombined in any suitable manner in one or more embodiments. In thefollowing description, numerous specific details are provided to give athorough understanding of embodiments. One skilled in the relevant artwill recognize, however, that the various embodiments can be practicedwithout one or more of the specific details, or with other methods,components, materials, et cetera. In other instances, well knownstructures, materials, or operations are not shown or described indetail to avoid obfuscation.

As used herein and in the appended claims, the singular forms “a,” “an”,and “the” include plural references unless the context clearly dictatesotherwise. Thus, for example, reference to “an element” includes aplurality of such elements and equivalents thereof known to thoseskilled in the art, and so forth, and reference to “the element” is areference to one or more such elements and equivalents thereof known tothose skilled in the art, and so forth. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range. Unlessotherwise indicated herein, and each separate value, as well asintermediate ranges, are incorporated into the specification as ifindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contraindicated by the text.

The terms “electronic circuitry”, “circuitry” or “circuit,” as usedherein include, but is not limited to, hardware, firmware, software orcombinations of each to perform a function(s) or an action(s). Forexample, based on a desired feature or need. a circuit may include asoftware controlled microprocessor, discrete logic such as anapplication specific integrated circuit (ASIC), or other programmedlogic device. A circuit may also be fully embodied as software. As usedherein, “circuit” is considered synonymous with “logic.” The term“logic”, as used herein includes, but is not limited to, hardware,firmware, software or combinations of each to perform a function(s) oran action(s), or to cause a function or action from another component.For example, based on a desired application or need, logic may include asoftware controlled microprocessor, discrete logic such as anapplication specific integrated circuit (ASIC), or other programmedlogic device. Logic may also be fully embodied as software.

The term “processor,” as used herein includes, but is not limited to,one or more of virtually any number of processor systems or stand-aloneprocessors, such as microprocessors, microcontrollers, centralprocessing units (CPUs), and digital signal processors (DSPs), in anycombination. The processor may be associated with various other circuitsthat support operation of the processor, such as random access memory(RAM), read-only memory (ROM), programmable read-only memory (PROM),erasable programmable read only memory (EPROM), clocks, decoders, memorycontrollers, or interrupt controllers, etc. These support circuits maybe internal or external to the processor or its associated electronicpackaging. The support circuits are in operative communication with theprocessor. The support circuits are not necessarily shown separate fromthe processor in block diagrams or other drawings.

The term “controller,” as used herein includes, but is not limited to,any circuit or device that coordinates and controls the operation of oneor more input and/or output devices. A controller may, for example,include a device having one or more processors, microprocessors, orcentral processing units capable of being programmed to performfunctions.

The term “logic,” as used herein includes, but is not limited to.hardware, firmware, software or combinations thereof to perform afunction(s) or an action(s), or to cause a function or action fromanother element or component. Based on a certain application or need,logic may, for example, include a software controlled microprocess,discrete logic such as an application specific integrated circuit(ASIC), or other programmed logic device. Logic may also be fullyembodied as software. As used herein, the term “logic” is consideredsynonymous with the term “circuit.”

The term “software,” as used herein includes, but is not limited to, oneor more computer readable or executable instructions that cause acomputer or other electronic device to perform functions, actions, orbehave in a desired manner. The instructions may be embodied in variousforms such as routines, algorithms, modules or programs includingseparate applications or code from dynamically linked libraries.Software may also be implemented in various forms such as a stand-aloneprogram, a function call, a servlet, an applet, instructions stored in amemory, part of an operating system or other type of executableinstructions. It will be appreciated by one of ordinary skill in the artthat the form of software is dependent on, for example, requirements ofa desired application, the environment it runs on, or the desires of adesigner/programmer or the like.

In a number of embodiment hereof, “trigger” or “sniffer” sensors hereofreduce power requirement and/or provide improved functionality ascompared to currently available combustible gas sensors. In a number ofembodiments hereof, a sensor having one or more low-thermal-masselements (for example, a sensing element and a compensating element)such as a Micro-Electro-Mechanical Systems or MEMS or micro-hotplatesensor is used to function as a trigger or sniffer sensor. A MEMShotplate sensor or other low-thermal-mass trigger sensor hereof may, forexample, provide a gas concentration reading. Alternatively the triggersensor may be used to simply measure a threshold value which triggersactivation of a primary, combustible gas sensor, which may be aconventional combustible gas sensor including pelements as describedabove or may include low-thermal-mass elements as described herein.

When the primary sensor is not activated, the primary sensor may be heldin a lower power (for example, a zero power) or OFF state. The primarysensor may, for example, be unable to sense a combustible gas analyte inthe OFF state. Upon activation of the primary sensor, the primary sensoris placed in a higher power state or ON state to sense the combustiblegas analyte and provide a more accurate reading of gas concentration.Whether the trigger sensor is used only to determine that a thresholdresponse value has been reached or the measured threshold response valueprovides a measure of concentration of a combustible gas analyte, themeasure of the threshold response value may cause an notification oralert to be issued to the user.

In general, operation of the trigger or sniffer sensor to activate aprimary sensor as described herein, reduces power requirements ascompared to operation of the primary sensor without the trigger orsniffer sensor. In a number of embodiments in which the trigger sensorprovides or outputs a concentration of a combustible gas analyte, theprimary sensor is configured to provide or output a more accurateconcentration reading and/or a concentration reading over a broaderrange of concentrations. Typically, but not necessarily, the element orelements of the trigger sensor hereof are of a lower thermal mass thanthe elements of the primary sensor. The trigger sensor may be continueto be activated or may be deactivated when the primary sensor isactivated.

MEMS elements have not previously been used in the function of a triggersensor. As further described below, low-thermal-mass,low-thermal-time-constant elements hereof may also be operated in anon-continuous or pulsed manner to further decrease power consumption ascompared to continuously powered trigger sensors. Moreover, in a numberof embodiments, a trigger sensor hereof may include both a sensingelement and a compensating element, which may, for example, provide fora compensated reading of a combustible analyte gas concentration fromthe trigger sensor. A user may, for example, be notified or alerted of aconcentration measure by the trigger sensor and/or if a measured valueof a trigger sensor response (which may, for example, be relatable to aconcentration) exceeds a threshold value.

Low thermal time constants associated with low thermal mass sensorsassist in providing quick response times, reducing the time an elementmay be unavailable for use in a detection mode and decrease powerrequirements. In a number of embodiments, trigger or sniffer sensorshereof (and/or low-thermal-mass primary sensors hereof) have an elementhaving a thermal time constant of 8 second or less, 6 seconds or less, 1second or less, 0.5 seconds or less or 0.250 second or less. A lowthermal mass/low thermal time constant sensor may, for example, includea MEMS pellistor as described above or a pelement of low thermal mass toprovide a thermal time constant. As used herein the thermal timeconstant of an element is defined as the time required to change 63.2%of the total difference between its initial and final temperature whensubjected to a step function change in drive power, under zero powerinitial conditions. MEMS pellistors typically have a lower thermal timeconstant than low-thermal-mass pelements. MEMS pellistors may, forexample, have thermal time constants of 1 second or less, 0.5 seconds orless or 0.250 second or less.

In general MEMS elements for sensors hereof have a dimension less than 1mm. Such element may be manufactured via a microfabrication technique.In a number of representative embodiments, sensing elements may bemanufactured with a thick film layer suitable to cause combustion of ananalyte gas upon heating to a predetermined temperature. Sensor elementshereof may be powered to an operating temperature by resistive heatingand to detect combustible gases analytes. In a number of representativeembodiments, the thickness and diameter for a MEMS hotplate oxidativefilm is 15 microns and 650 microns, respectively.

In a number of embodiments, beads or pelements of low thermal mass andhaving low thermal constants as described above may be used as triggersensor elements as well as primary sensor element hereof.Low-thermal-mass/low-thermal-time constant pelements are, for example,discussed in U.S. Pat. No. 8,826,721, the disclosure of which isincorporated herein by reference. Such pelements may, for example, havea diameter less than 500 μm or have a volume less than a sphere having adiameter 500 μm. In general, trigger sensors hereof need only sufficientthermal mass to generate a measurable signal in the presence of ananalyte gas.

Oxidative layers of trigger elements hereof may, but need not, include acatalyst. In the case that a catalyst is used the oxidative layer maycause combustion of a combustible gas analyte at a lower temperature(for example, in the range of 100 to 700° C.) than in the absence of acatalyst. For example, a temperature above 700° C. or above 900° C. maybe required to induce oxidation/combustion of the combustible gasanalyte in connection with an element not including a catalyst.

Conventional catalytic combustible gas sensor elements and/orlow-thermal-mass sensor elements hereof may be operated in a Wheatstonebridge as, for example, described in connection with FIG. 1B. Primary ortrigger sensors hereof may, for example, be operated in constantcurrent, constant voltage, or constant resistance. As described above,catalytic elements of a combustible gas sensor are operated in, forexample, a temperature range of 100 to 700° C. whenever the sensor isoperational. A mode of operation of a sensor at constant current or atconstant voltage may be termed a “continuous” mode of operation.

An alternate operational mode, which is particularly suitable forlow-thermal-mass pelements such as MEMS hotplates/pellistors, is toquickly heat and cool a sensor elements or elements in a pulsed powermode. An advantage to operating in pulse mode is significantly lowerpower consumption as compared to continuous mode. Another advantage isimproved span response as a result of adsorption of excess combustiblegas on the catalyst at cooler temperatures during unpowered or lowerpowered operation (that is, during a REST time) as compared tocontinuously powering the catalyst at the run temperature of 100 to 700°C.

Pulse width modulation may, for example, be used to control the energydelivered to the heating element(s) of the trigger and primary sensorshereof. Pulse width modulation is a well-known control technique used tocontrol the average power and/or energy delivered to a load. Inembodiments hereof, a voltage is supplied to, for example, a MEMShotplate or other heating element to heat the oxidative layer andsupported catalyst/structure, when present, to a desired temperature.Because the trigger sensors hereof have relatively low thermal mass, thecycle times can be relatively short.

In a pulse mode of operation, heating energy (that is, heatingvoltage(s) or heating currents(s)) may be periodically supplied to theheating element(s) during an “ON time” in a “pulse mode”. Rest energy(that is, rest voltage(s) or a rest current(s)), which is less than theheating energy may be supplied during a “REST time”. The total of thehigher-energy or ON time plus the lower-energy or REST time correspondto a cycle time or a cycle duration. Gas concentration of the analyte ismeasured during the ON time. The heating energy (voltages/currents)supplied during the ON time may be constant during the ON time or may bevaried (for example, supplied as heating voltage/current plateau or asheating voltage/current ramp). The rest energy (voltages/currents) maybe equal to zero, or be sufficiently lower than the heating energy sothat the trigger gas sensor does not interact in a significant mannerwith (or consume any or substantially any) gas to be detected. Similarto the ON time, the rest energy supplied during the REST time may beconstant during all of the REST time or may be varied (for example,supplied as rest voltage/current plateau or as rest voltage/currentramp). The cycle may be repeated.

The ON time duration may, for example, be in the range of 100 msec to 1second or in the range of 300 msec to 500 msec in a number ofembodiments hereof. In a number of embodiments, the ON time duration maybe kept as short as possible to improve response time. In a number ofembodiments, the duty cycle may, for example, be in the range of 5% to12% (ratio of ON time/(ON time+REST time). In a number of embodiments,the ON time is approximately 350 msec (that is, equal to or within 10%of that) and the duty cycle is approximately 10% (that is, equal to orwithin 20% of that value). In a representative example, the cycle timeor cycle duration was 4000 msec, during which the ON time was 350 msecand the REST time was 3650 msec. Therefore, the duty cycle is 8.75%.

FIG. 2A illustrates a cutaway view of an embodiment of a MEMS ormicro-hotplate trigger sensor hereof, which includes a housing 102having a gas inlet 110. A screen or cap 120, which may include orfunction as a filter 130, may, for example, be placed in connection withinlet 110. The energy (current and voltage) used in MEMS micro-hotplatetrigger sensor 100 may, for example, be sufficiently low to provideintrinsic safety such that a flashback arrestor, as known in thecombustible gas detector arts, may not be necessary. As described above,flashback arrestors (for example, porous frits) allow ambient gases topass into a housing but prevent ignition of combustible/flammable gas inthe surrounding environment by hot elements within the housing. One ormore heating elements or hotplates 140 may, for example, be used to heatan oxidative layer 152 (which may, for example, be an oxidative catalystlayer) of a first MEMS element or pellistor 150 to a first operatingtemperature. In a number of embodiments, a second MEMS element or secondpellistor 150′ may be included within MEMS hotplate trigger sensor 100to be heated to a second operating temperature.

In a number of embodiments, first MEMS element 150 may be operated as asensing or detecting element and second MEMS element 150′ may beoperated as a compensating element as known in the combustible gassensor arts. In other embodiments, as further described below, theoperation of MEMS elements 150 and 150′ may be switched by altering themode of operation thereof.

In a number of embodiments, the operation of a particular element as asensing element or a compensating element may be controlled bycontrolling the operating temperature thereof. If the operatingtemperature of an element is maintained at or above a temperature atwhich gas will combust at the surface thereof, it may be operated as asensing element. If the operating temperature of an element ismaintained below a temperature at which gas will combust at the surfacethereof, it may be operated as a compensating element. The temperatureat which gas will combust at the surface of an element depends upon thecomposition of that surface. Surfaces including a catalytic materialwill typically cause combustion at a temperature (a catalytic light-offtemperature) lower than a surface not including a catalytic material.

If operated solely as a MEMS compensator element 150′ may, for example,include an inactive layer 152′ which may be heated by one or moreheating elements or hotplates 140′. In this case, the second operatingtemperature may be maintained at a temperature lower than thetemperature required to cause combustion at a surface thereof in theabsence of a catalyst. Alternatively layer 152′ may include an activecatalyst and be operated at a sufficiently low temperature to preventcatalytic oxidation of combustible gas at the surface thereof.

MEMS hotplate sensor 100 may, for example, mounted on a printed circuitboard or PCB 200. The two resistances of the sensing element 150 and thecompensating element may, for example, be part of a measurement circuitsuch as a Wheatstone bridge circuit as illustrated in FIG. 1C or asimulated Wheatstone bridge circuit. A representative example of a MEMShotplate sensor suitable for use herein is a SGX MP7217 hotplate sensoror pellistor available from SGX Sensortech, SA ofCorcelles-Coromondreche, Switzerland. Such a MEMS hotplate sensor isdisclosed, for example, in U.S. Pat. No. 9,228,967, the disclosure ofwhich is incorporated herein by reference.

The devices and systems hereof may, for example, be deployed either as atwo-sensor system or the low-thermal-mass trigger sensor can be embeddedinto a single combustible gas sensor that, for example, includes aprimary combustible gas sensor, which provides a calibrated responsewhich is dependent upon analyte gas concentration. The primarycombustible gas sensor may, for example, be a traditional orconventional catalytic bead system, a lower-thermal-mass pelement systemor a low-thermal-mass MEMS system. FIG. 2C schematically illustrates anembodiment of a sensor system or device 10 a hereof which includes aMEMS hotplate combustible gas sensor 100 as a trigger combustible gassensor and a conventional combustible gas sensor 300 as a primarycombustible gas sensor. As illustrated in FIG. 2C, MEMS hotplatecombustible gas sensor 100 and combustible gas sensor 300 may, forexample, be placed in operative connection with a control system 400 anda power source 500 (for example, a battery system including one or morebatteries) via PCB 200.

The electronic circuitry of sensor system or device 10 a may, forexample, be in operative connection with each of MEMS micro-hotplate,trigger combustible gas sensor 100 and primary combustible gas sensor300 to, for example, control power to the sensors (as described above)and to process an output signal from the sensors. The electroniccircuitry of sensor system 10 may, for example, include or be inoperative connection with a control system or controller including aprocessor system 410 (including, for example, one or more processorssuch as microprocessors) and a memory system 420. One or more algorithmsfor control of sensor 100 and sensor 300 and/or for processing of datamay, for example, be stored in memory system 420, which is in operativeconnection with processor system 430. Output of sensors 100 and/or 300may, for example, be provided to a user or users via a user interface430 (for example, including one or more devices for input/output ofinformation including a touch screen display, a speaker etc.) inoperative connection with processor system 410. A user interface 430may, for example, be provided as a component of the combustible gassensor system or device 10 a and/or remote from the combustible gassensor. An alarm signal may, for example, be generated via theelectronic circuitry and provided to a user via one or more componentsof user interface 430 (for example, visually, audibly etc.).

As described above, for combustible gas sensor system of device 10,primary combustible gas sensor 300 includes a first element or pelement340 and second element or pelement 350. As described above, one of thefirst element 340 and second element 350 is operated as a sensingelement, while the other of first element 340 and second element 350 isoperated as a compensating element. Once again, the function of anelement can, for example, be controlled by the mode ofoperation/operating temperature thereof. The temperature may becontrolled by control of the power provided to the element. The tworesistances may, for example, be part of a measurement circuit such as aWheatstone bridge circuit as illustrated in FIG. 1C. The characteristicsof second pelement 350 may be matched as closely as possible with activeor sensing pelement 340. In a number of embodiments in which secondelement 350 is operated solely as a compensating pelement, secondelement 350 may, for example, either carry no catalyst or carry aninactivated or poisoned catalyst.

In the illustrated embodiment, first element or pelement 340 and secondelement or pelement 350 are positioned within wells 360 a and 360 b ofan explosion-proof housing 370 and can be separated from the surroundingenvironment by a flashback arrestor, for example, a porous metal frit380. A filter element 382 may also be present.

In operation of system or device 10 a in a first mode, MEMS hotplatetrigger sensor 100 is operated to detect a change in the analyte gas orgases entering system or device 10 a, while primary combustible gassensor 300 is maintained in a low power, inactive, or OFF state. If aresponse signal from MEMS hotplate trigger sensor 100 is determined bycontrol system 200 to indicate that the concentration of one or moreanalyte gases has changed or has changed in an amount above apredetermined threshold, a second mode of operation is initiated andactivation of primary combustible gas sensor 300 occurs (that is, apowered, active or ON state of primary combustible gas sensor 300 isactivated).

As described above, trigger sensors hereof need not be, but may be“diagnostic” sensors with sufficient characteristics to provide anaccurate indication of the concentration of the combustible gas analytebeing sensed. Such characteristics would include, for example,sufficient response range to provide accurate indications of gas contentover a desired range of concentration, long-term baseline stability,significant resistance to changes arising from environmental conditions,etc. The trigger sensors hereof may be “non-diagnostic” or“pseudo-diagnostic.” In that regard, the trigger sensors may havesufficient range and accuracy to be useful to accomplish the triggerfunction described herein. Stability and accuracy are not as importantin this functionality inasmuch as non-conservative false negativedeviations are avoided. In a number of embodiments, there is no need torefer to an earlier established calibration event in the case ofnon-diagnostic trigger sensor hereof. In the case of a non-diagnostictrigger sensor, a compensating element may not be required.

Providing both a first MEMS or other low-thermal-mass element and asecond MEMS or other low-thermal-mass element in a trigger sensor hereofrequires very little energy for operation. Unlike previous attempts toprovide a trigger sensor or monitor in which only nondiagnostic ornonspecific changes in gas concentration are detected by a singletrigger bead, the combustible gas, trigger sensors hereof may provide arelatively accurate output of gas concentration. In that regard, theMEMS or other low-thermal-mass combustible gas trigger sensors hereofprovide a compensated gas concentration output or reading that may beposted to a user while the primary combustible sensor (for example, apelement-based sensor 300) is inactive and/or when activated and warmingup sufficiently to take a reading. Moreover, a reading or measurementfrom the a micro-hotplate or other low-thermal-mass trigger sensorhereof may be used to activate an alarm level if needed.

Even in the case in which a trigger sensor hereof include only a singleelement, use of a MEMS element provides significant reductions in powerrequirements as compared to currently available trigger beads. Moreover,whether a trigger sensor hereof include one or two elements, significantreductions in power requirements may be achieved by operation in apulsed mode.

As described above, providing a low-power trigger sensor can reducerequired power by allowing very low power (including zero power)operation of a primary, higher-power, analytical sensor until thesecondary, trigger sensor detects a change or a threshold change inconcentration of a combustible gas. Operating a trigger sensor in apulse mode, as described herein, provides an intermediate monitoring ofthe gas mixture of the environment in fluid connection with the sensordevice, system or instrument.

As also described above, the first primary element 340 and secondprimary element 350 may be operated in a cyclic mode as, for example,disclosed in U.S. Pat. Nos. 8,826,721 and 9,625,406. In that regard, asensor may be cycled between a first mode in which first primary element340 is operated in a higher power mode and second primary element 350 isoperated in a lower power mode and a second mode in which second orcompensating element is operated in a higher power mode and the first orsensing element is operated in a lower power mode. In that regard, theelectronic circuitry of the system hereof may, for example, be adaptedto or configured to cycle between a first mode in which the firstprimary element is operated in a higher power (high temperature) modeand the second sensing element is operated in a lower power (lowertemperature) mode and a second mode in which the second sensing elementis operated in a higher power (higher temperature) mode and the firstsensing element is operated in a lower power (lower temperature) mode.In the first mode, the second sensing element can, for example, be usedto compensate for ambient temperature changes. In the second mode, thefirst sensing element can, for example, be used to compensate forambient temperature changes. The electronic circuitry can, for example,be adapted to periodically switch between the first mode and the secondmode. The electronic circuitry can, for example, be adapted to switchbetween the first mode and the second mode upon a manually controlledevent. The manually controlled event can, for example, include a poweron event.

Similarly, first trigger element 140 and second trigger element may beoperated in a cyclic mode as disclosed in U.S. Pat. Nos. 8,826,721 and9,625,406. In that regard, one may cycle between a first mode in whichfirst trigger element 150 is operated in a higher power mode and secondtrigger element 150′ is operated in a lower power mode and a second modein which second trigger element 150′ is operated in a higher power modeand first trigger element 150 is operated in a lower power mode.

Testing has, for example, been conducted wherein elements were operatedat relatively low currents (for example, as low as 1/20 of the normaloperating current) to monitor the effect of changes in ambienttemperature only. In such an operating mode, the elements require, forexample, only a few milliwatts or less to operate. The results of suchtesting indicate that elements operated at low power (that is,sufficiently low power to reduce the catalytic or other activity of theelement, for example, to render the element substantially or completelyinactive with respect to combustible gas), can be used as a replacementfor a traditional compensating element. Since pressure effects arenegligible and only wire chemistry temperature effects are significantin the element, the need to match such sensing/compensating elementsprecisely is significantly relaxed.

Elements of sensors hereof may be switched between a high power/hightemperature active mode and a low power/low temperature inactive orcompensating mode over a wide range of periods. In general, the periodof cycling is limited (on the lower end) by the amount of time requiredto achieve equilibrium or steady state operation (that is, by thethermal time constants of the element). Electronic circuitry hereof,can, for example, effect automatic periodic switching between sensingelement modes.

Other power/temperature control modes for combustible gas sensorelements which may, for example, be used to limit catalyst inhibitionare disclosed in U.S. patent application Ser. Nos. 15/597,933 and15/597,859, the disclosures of which are incorporated herein byreference. Such methodologies may be readily adapted for use herein.

U.S. Pat. Nos. 8,826,721, 4,533,520 and 5,780,715 disclose systems andmethods to, for example, balance bridges electronically and to operatesensing elements independently. Such systems and methods can, forexample, be incorporated in circuitry hereof.

The foregoing description and accompanying drawings set forth a numberof representative embodiments at the present time. Variousmodifications, additions and alternative designs will, of course, becomeapparent to those skilled in the art in light of the foregoing teachingswithout departing from the scope hereof, which is indicated by thefollowing claims rather than by the foregoing description. All changesand variations that fall within the meaning and range of equivalency ofthe claims are to be embraced within their scope.

What is claimed is:
 1. A sensor system, comprising: electronic circuitrycomprising a control system; a primary combustible gas sensor comprisinga first primary element in operative connection with the electroniccircuitry and comprising a first primary support structure, a firstprimary catalyst supported on the first primary support structure and afirst primary heating element in operative connection with the firstprimary support structure; a second primary element in operativeconnection with the electronic circuitry and comprising a second primarysupport structure, a second primary catalyst supported on the secondprimary support structure and a second primary heating element inoperative connection with the second primary support structure; and atrigger combustible gas sensor comprising a first trigger element oflow-thermal-mass comprising a first trigger heating element, the firsttrigger element being in operative connection with the electroniccircuitry, and a second trigger element of low thermal mass comprising asecond trigger heating element, the second trigger element being inoperative connection with the electronic circuitry, the electroniccircuitry having a first trigger mode of operating in which the firsttrigger element is heated to a temperature at or above a temperature atwhich the first trigger element causes combustion of at least onecombustible gas analyte and wherein the second trigger element isoperated as a trigger compensating element; the electronic circuitryconfigured to operate the trigger combustible gas sensor to detect avalue of a response at or above a threshold value, the primarycombustible gas sensor being activated from a low-power state upon thethreshold value being detected by the trigger combustible gas sensor. 2.The sensor system of claim 1 wherein the electronic circuitry has afirst primary mode of operating during activation thereof in which thefirst primary element is heated above a temperature at which the firstprimary catalyst catalyzes combustion of the at least one combustiblegas analyte and wherein the second primary element is operated as aprimary compensating element
 3. The sensor system of claim 2 wherein thesecond primary element is operated at a lower power than the firstprimary element in the first primary mode.
 4. The sensor system of claim3 wherein the electronic circuitry has a second primary mode ofoperating in which the second primary element is heated above atemperature at which the second primary catalyst catalyzes combustion ofat least one combustible gas analyte and wherein the first primaryelement is operated as a primary compensating element.
 5. The sensorsystem of claim 4 wherein the first primary element is operated at alower power than the second primary element in the second primary mode.6. The sensor system of claim 1 wherein the second trigger element isoperated at a lower power than the first trigger element in the firsttrigger mode.
 7. The sensor system of claim 6 wherein the electroniccircuitry has a second trigger mode of operating in which the secondtrigger element is heated above a temperature at which the secondtrigger element causes combustion of the at least one combustible gasanalyte and wherein the first trigger element is operated as a triggercompensating element.
 8. The sensor system of claim 7 wherein the firsttrigger element is operated at a lower power than the second triggerelement in the second trigger mode.
 9. The sensor system of claim 1wherein the first trigger element comprises a first trigger catalyst andthe second trigger element comprises a second trigger catalyst.
 10. Thesensor system of claim 8 wherein the first trigger element comprises afirst trigger catalyst and the second trigger element comprises a secondtrigger catalyst.
 11. The sensor system of claim 1 wherein the firsttrigger element comprises a first MEMS element and the second triggerelement comprises a second MEMS element.
 12. The sensor system of claim1 wherein the value of the response of the trigger combustible gassensor is a concentration of the at least one combustible gas analytewhich is output via a user interface in operative connection with thecontrol system.
 13. The sensor system of claim 1 wherein during thefirst trigger mode the electronic circuitry is configured toperiodically cycle the first trigger element between the temperature ator above which the first trigger element causes combustion of the atleast one combustible gas analyte and another temperature at which thefirst trigger element is substantially inactive to cause combustion ofthe at least one combustible gas analyte.
 14. The sensor system of claim7 wherein during the second trigger mode the electronic circuitry isconfigured to periodically cycle the second trigger element between thetemperature at or above which the second trigger element causescombustion of the at least one combustible gas analyte and anothertemperature at which the second trigger element is substantiallyinactive to cause combustion of the at least one combustible gasanalyte.
 15. The sensor system of claim 1 wherein a user is notified viaa user interface in operative connection with the control system whenthe value of the response of the trigger combustible gas sensor is at orabove the threshold value.
 16. The sensor system of claim 1 wherein eachof the first trigger element and the second trigger elementindependently has a thermal time constant no greater than 8 seconds. 17.The sensor system of claim 1 wherein each of the first trigger elementand the second trigger element independently has a thermal time constantno greater than 1 second.
 18. The sensor system of claim 1 wherein eachof the first trigger element and the second trigger elementindependently has a thermal time constant no greater than 0.250 seconds.19. A sensor system, comprising: electronic circuitry comprising acontrol system; a primary combustible gas sensor comprising a firstprimary element in operative connection with the electronic circuitryand comprising a first primary support structure, a first primarycatalyst supported on the first primary support structure and a firstprimary heating element in operative connection with the first primarysupport structure; a second primary element in operative connection withthe electronic circuitry and comprising a second primary supportstructure, a second primary catalyst supported on the second primarysupport structure and a second primary heating element in operativeconnection with the second primary support structure; and a triggercombustible gas sensor comprising a first MEMS trigger elementcomprising a first trigger heating element, the first trigger MEMSelement being in operative connection with the electronic circuitry, theelectronic circuitry being configured to heat the first MEMS triggerelement to at temperature at or above a temperature at which the firstMEMS trigger element causes combustion of at least one combustible gasanalyte; the electronic circuitry configured to operate the triggercombustible gas sensor to detect a value of a response at or above athreshold value, the primary combustible gas sensor being activated froma low-power state upon the threshold value being detected by the triggercombustible gas sensor.
 20. The sensor system of claim 19 wherein theelectronic circuitry has a first primary mode of operating in which thefirst primary element is heated to a temperature at or above atemperature at which the first primary catalyst catalyzes combustion ofthe at least one combustible gas analyte and wherein the second primaryelement is operated as a primary compensating element;
 21. The sensorsystem of claim 19 wherein the trigger combustible gas sensor furthercomprises a second MEMS trigger element comprising a second triggerheating element, the second MEMS trigger element being in operativeconnection with the electronic circuitry, wherein the second MEMStrigger element is operated as a compensating element in at least onemode of operation of the sensor system.
 22. The sensor system of claim19 wherein the electronic circuitry is configured to periodically cyclethe first trigger element between the temperature at or above which thefirst trigger element causes combustion of the at least one combustiblegas analyte and another temperature at which the first trigger elementis substantially inactive to cause combustion of the at least onecombustible gas analyte.
 23. The sensor system of claim 21 wherein theelectronic circuitry is configured to periodically cycle the firsttrigger element between the temperature at or above which the firsttrigger element causes combustion of the at least one combustible gasanalyte and another temperature at which the first trigger element issubstantially inactive to cause combustion of the at least onecombustible gas analyte.
 24. The sensor system of claim 21 wherein eachof the first MEMS trigger element and the second MEMS trigger elementindependently has a thermal time constant no greater than 1 second. 25.The sensor system of claim 21 wherein each of the first MEMS triggerelement and the second MEMS trigger element independently has a thermaltime constant no greater than 0.500 seconds.
 26. The sensor system ofclaim 21 wherein each of the first MEMS trigger element and the secondMEMS trigger element independently has a thermal time constant nogreater than 0.250 seconds.